Research Article A Parallel-Strip Balun For Wideband

Hindawi Publishing CorporationInternational Journal of Microwave Science and TechnologyVolume 2013, Article ID 892590, 4 pageshttp://dx.doi.org/10.1155/2013/892590Research ArticleA Parallel-Strip Balun for Wideband Frequency DoublerLeung Chiu and Quan XueDepartment of Electronic Engineering, City University of Hong Kong, Hong KongCorrespondence should be addressed to Leung Chiu; eechiuleung@yahoo.com.hkReceived 18 September 2013; Revised 2 November 2013; Accepted 9 November 2013Academic Editor: Yong Xin GuoCopyright 2013 L. Chiu and Q. Xue. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.A parallel-strip phase inverter with a pair of simple impedance matching networks is designed. The phase inverter introduces thealmost frequency independent 180 phase shift and was employed in the wideband parallel-strip balun. The balun was designed andmeasured with the maximum magnitude imbalance of 0.5 dB and the maximum phase imbalance of 6.0 . The proposed balun is usedas input network for the wideband balanced frequency doubler. The proposed frequency doubler achieves significant conversiongain from 0.1 GHz to 1.7 GHz. The frequency doubler achieves 7.4 dB conversion gain and 23 dB fundamental signal suppression at1 GHz.1. IntroductionDifferential radio frequency (RF) circuits are commonlyfound in the integrated circuits for wireless communicationsystems. The RF ports of these chips are not standalone, butthey consist of both “positive” and “negative” terminals. However, most of the off-chip RF components such as antennaand switch are single-ended [1]. Balun, that, is a device forconversion between a single-ended signal and differentialsignals, is essential for the entire wireless module [2]. The differential signals are composed of two separated signals equalin magnitude but 180 out-of-phase. Differential amplifiersand differential oscillators outperform single-ended circuitsin terms of the even-order harmonic signal and commonmode noise suppressions [3]. A high performance balundesign is critical for integrating the single-end and differentialcircuit in an efficient way. Besides, impedance bandwidth,phase imbalance, and magnitude imbalance are the threeimportant design issues for the balun design.Wilkinson power divider divides signal with high portto-port isolation, and the impedance of all ports is matched[4]. However, the divided signals are in-phase. Additional andfixed 180 phase shifter is required for the balun based on theWilkinson power divider. The frequency responses of bothinsertion loss and phase flatness of the phase shifter are keysof the balun performance. Many high performance balundesigns based on the Wilkinson power divider were reportedin [1, 4]. In [1], the 180 phase shifter was based on the combination of open circuit and short circuit stubs and microstriplines. The measured impedance bandwidth is about 64%.Another work based on microstrip metamaterial linesthat was designed using lumped elements achieved impedance bandwidth of about 77% [4].A parallel-strip line is a balanced transmission line, whichconsists of two conductors separated by a dielectric substrate.The parallel-strip line achieves a wide range of characteristicimpedance lines and high performance balanced microwavecomponents [5]. The parallel-strip phase inverter is realizedby interconnecting the upper and lower conductors of a PCBthrough the two metal vias. This phase inverter introducesa very wideband 180 phase shift. The parallel-strip inverterwas successfully applied to filter and directional couplerdesigns with performance enhancement [6, 7].In the balanced frequency doubler design, the twoidentical nonlinear devices such as biased transistors anddiodes were fed by a differential signal that is generatedfrom the balun by single signal source. The output signalsfrom the two devices are combined by in-phase powercombiner. Meanwhile, the even-order harmonic signals arecombined and extracted at the output port, and the oddorder harmonics signals are suppressed [8, 9]. However, thebandwidth of a frequency doubler is limited by the balun.In this paper, a wideband balun based on Wilkinson powerdivider and parallel-strip inverter is proposed. Besides, a

2International Journal of Microwave Science and TechnologyPhase inverterPort 2(50 Ω)WilkinsonPort 1Port 2powerViadividerPort 1(50 Ω)Port 3(50 Ω)ResistorPort 3Delay line(a)(b)5270 5422531802135190045 10 15 20 25 3000.511.52Frequency (GHz)S21S312.53S11S32(a) 100.511.52Frequency (GHz)2.53Phase error (deg)0Magnitude error (dB)S-parameters (dB)Figure 1: (a) Circuit diagram of the parallel-strip balun. (b) 3D view of the parallel-strip balun.0Magnitude errorPhase error(b)Figure 2: (a) Measured frequency responses of the magnitudes of the 𝑆-parameters of the balun. (b) Measured magnitude error and phaseerror of the balun.wideband frequency doubler based on the proposed balun ispresented.2. Parallel-Strip BalunThe proposed balun consists of three parts, a parallel-stripWilkinson power divider, a parallel-strip phase inverter, anda section of delay parallel-strip line. The circuit diagramand the geometry of the balun are shown in Figure 1.The proposed balun is designed at the centre frequency of1.0 GHz and it is simulated by the full-wave electromagneticsimulation software, Zeland IE3D [10]. The parallel-stripphase inverter is the core of the proposed balun and itintroduces an approximately 180 to the one of the dividedsignal. The design and the application of the parallel-stripinverter are reported in [6, 7]. There are two lumped 50 Ωisolation resistors mounted on both the upper and lowerlayers [5]. Figure 2 shows its measured frequency responsesof the magnitudes of the four 𝑆-parameters. The measuredbandwidth with full considerations of impedance matching 𝑆11 , 𝑆22 , and 𝑆33 10 dB and isolation bandwidth 𝑆32 10 dB is about 100% (0.48 GHz–1.44 GHz). Withinthe above frequency range, the maximum magnitude error 𝑆12 /𝑆13 (dB) and the maximum phase error 𝑆12 - 𝑆13 -180 are 0.50 dB and 6.0 , respectively.3. Proposed Frequency DoublerFigure 3 shows the structure and the photograph of thebalanced frequency doubler. It consists of two identicalnonlinear devices, one input balun and one output Wilkinsonpower combiner. The biased bipolar junction transistor incommon emitter configuration is closed for the nonlineardevice since it achieves conversion gain. Diode can beused, but conversion loss is achieved instead of conversiongain. The conversion gain is maximized if Class-B biasingcondition of the two transistors is chosen. The two nonlineardevices are governed by𝑦1 𝑎1 𝑥1 𝑎2 𝑥12 𝑎3 𝑥13 𝑎4 𝑥14 𝑎5 𝑥15 ,𝑦2 𝑎1 𝑥2 𝑎2 𝑥22 𝑎3 𝑥23 𝑎4 𝑥24 𝑎5 𝑥25 ,(1)where 𝑥1 and 𝑥2 are the inputs of the two nonlinear devicesand 𝑦1 and 𝑦2 are the outputs of the two nonlinear devices.

International Journal of Microwave Science and TechnologyNonlineardevicesBalun3Wilkinson powercombinerInputportTapered line 545401035530025 52015 1010 15 20Fundamental suppression (dB)Conversion gain (dB)Figure 3: (a) Top view of the proposed frequency doubler. (b) Bottom view of the proposed frequency doubler.500.40.81.2Input frequency (GHz)1.620Conversion gainFundamental suppressionFigure 4: Frequency response of the conversion gain and fundamental suppression with fixed input power of 30 dBm and fixedbiasing condition.The input balun divides the input signal into two equal inmagnitude but 180 out-of-phase signals that are the inputsof the two nonlinear devices; namely, 𝑥2 𝑥1 . The outputsof two nonlinear devices become𝑦1 𝑎1 𝑥1 𝑎2 𝑥12 𝑎3 𝑥13 𝑎4 𝑥14 𝑎5 𝑥15 ,2𝑦2 𝑎1 ( 𝑥1 ) 𝑎2 ( 𝑥1 ) 𝑎3 ( 𝑥1 )434. Conclusion(2)5 𝑎4 ( 𝑥1 ) 𝑎5 ( 𝑥1 ) .The in-phase Wilkinson power combiner is used to combinethe two outputs into one. The eventual output becomes𝑦 𝑦1 𝑦2 ,𝑦 𝑎2 𝑥12 𝑎4 𝑥14 𝑎6 𝑥16 .The fundamental signal and all the odd-order harmonic signals are cancelled in the ideal case. Practically, these signalsare not completely suppressed since the balun always introduces magnitude and phase errors.In this study, a wideband frequency doubler is proposedusing the parallel-strip balun. The output Wilkinson powercombiner achieves wide impedance bandwidth. The proposed frequency doubler was fabricated on the dielectricsubstrate with a relative permittivity of 4.6 and substratethickness of 1.0 mm. Impedance matching networks couldbe used to enhance the conversion gain at certain narrowrange of frequency, since impedance matching network isfrequency dependent. The network is the critical factor tolimit the bandwidth of the frequency doubler. In this study, nomatching networks are designed for the transistors to ensurethe frequency doubler working over wide frequency range.Figure 4 shows the frequency response of the conversiongain with fixed input power of 30 dBm and fixed biasingcondition. The significant conversion gain is achieved overinput frequency from 0.1 GHz to 1.7 GHz (178% relativebandwidth). The input power is fixed at 3 dBm. It achieves7.4 dB conversion gain and 23 dB fundamental suppressionat 1.0 GHz. Figures 5 and 6 show the measured conversiongains and the measured supply currents against the input fundamental power up to 3 dBm, respectively. Three differentinput signal frequencies, 0.8 GHz, 1.0 GHz, and 1.2 GHz, arechosen, and the output frequencies are 1.6 GHz, 2.0 GHz, and2.4 GHz, respectively.(3)The parallel-strip phase inverter with simple and compactimpedance matching networks is designed. The parallelstrip balun consisting of a Wilkinson power divider, a phaseinverter, and a section of delay line is achieved with widebanddifferential output. The balanced frequency doubler using theproposed balun was designed and fabricated on a single pieceof printed circuit board. A wideband frequency doubler basedon the parallel-strip circuit is demonstrated for the first time.The proposed balun can also be integrated to other balanceddevices such as microwave mixers, oscillator, and antennas.